Vehicle systems may include an engine with an exhaust gas treatment system coupled in an exhaust passage in order to control regulated emissions. In some examples, the exhaust gas treatment system may include a selective catalytic reduction (SCR) system in which a reductant, such as urea or ammonia, is added to the exhaust stream upstream of a reduction catalyst such that NOx may be reduced by the catalyst. The SCR system may also include one or more NOx sensors such as a feedgas NOx sensor coupled upstream of the SCR catalyst and a tailpipe NOx sensor coupled downstream of the SCR catalyst. Based on the output of the upstream and downstream NOx sensors, a performance of the SCR catalyst may be determined. In addition, dosing control of the reductant may be adapted based on the output of the NOx sensors. Therefore, to enable accurate dosing control as well as to enable monitoring of the SCR system efficiency, the sensors may need to be periodically diagnosed.
Thus, methods and systems for diagnosing of a feedgas exhaust NOx sensor coupled in an exhaust passage upstream of an exhaust SCR catalyst is provided. One example method comprises indicating degradation of a feedgas exhaust NOx sensor based on an exhaust reductant level estimated by the sensor following engine shutdown to rest. In this way, NOx sensor health can be correlated with the lingering presence of ammonia deposits after a vehicle engine has been turned off.
For example, an engine system may be configured with an SCR catalyst in the exhaust passage and a urea injector positioned upstream of the SCR catalyst. A feedgas NOx sensor may be coupled to the exhaust passage upstream of the SCR catalyst and downstream of the urea injector. After an engine shutdown to rest, a controller may operate a reductant injector to inject a defined amount of reductant into the exhaust passage. The controller may then monitor the response of the feedgas NOx sensor. If the output of the NOx sensor does not match an output expected based on the active injection of reductant, NOx sensor degradation may be determined. Based on the deviation of the estimated output from the expected output, dynamic characteristics of the feedgas NOx sensor may be learned and updated so that reductant dosing control can be adjusted during a subsequent engine restart.
In this way, the health and performance characteristics of a feedgas exhaust NOx sensor can be better identified. By monitoring the output of an exhaust NOx sensor during engine shutdown conditions, while reductant is injected upstream of the sensor, correlations between the injection and the exhaust NOx sensor output can be used to learn NOx sensor behavior. Specifically, natural sublimation of ammonia injected in an exhaust passage after an engine shutdown can be used to diagnose an exhaust NOx sensor. By improving NOx sensor diagnostics, emissions compliance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.